Field of the Invention
The invention relates to a read-only memory and to a method for fabricating the memory.
Read-only memories are widely referred to as ROMs. They are nonvolatile memories which cannot be erased. The so-called mask ROM is widespread in practice. The read-only memory is fabricated as an integrated circuit whose individual layers are fabricated by means of lithographic projection methods. In order to carry out a lithographic projection method, first of all a resist layer is applied to a planar layer of the integrated circuit. Afterwards, through a corresponding mask, the entire resist layer is exposed simultaneously at the locations left free by the mask. The mask is then removed and the resist layer is developed and its exposed parts are removed. The remaining parts of the resist layer are subsequently used for further processing of the integrated circuit, for example for an etching process or for a dopant implantation process.
The lithographic projection method outlined above is the most frequently used lithographic method for the fabrication of integrated circuits. Owing to the whole-area exposure (projection) of the resist layer, that method requires the production of the masks outlined above. Once the masks are available, however, they can be used to fabricate a large number of identical integrated circuits in a short period of time.
In addition to the lithographic projection methods outlined above, lithographic beam writing methods are also known. There, the exposure of a resist layer is effected by means of an electron beam or an ion beam. Such a beam which can be controlled by electric fields enables very fine structures to be fabricated and makes it unnecessary to produce masks of the kind necessary for the lithographic projection methods. The disadvantage of the beam writing methods is that the desired exposure of the resist layer is significantly more timeconsuming than in the case of the exposure carried out over the full area in the projection methods. Therefore, the lithographic beam writing methods are used only for fabricating very small structures within integrated circuits that are fabricated in small numbers.
Read-only memories are frequently used in smart cards, where they serve as memories for a processor likewise disposed in the smart card. For example, the microcode required by the processor is then stored in the ROM. In many applications (for example in a credit card or charge card context), it is additionally necessary, for security reasons, to store an individual identifier (for example a PIN number) in each smart card. Since the customary mask programming of the ROM does not permit storage of individual data for each smart card in the ROM, the individual identifier is stored in an EEPROM (Electrically Erasable Programmable ROM) that is additionally provided in the smart card for this purpose.
Due to the fact, however, that it can be erased, an EEPROM has the disadvantage that manipulation of the security-relevant data stored in it cannot be precluded.